engine: Fix up some send/recv corner cases

Initially I wasn't 100% clear or decided on the send/recv semantics.
After some experimenting, I think this is much closer to what we want.
Nothing should break or regress here, this only enables more
possibilities.

examples/lang/sendrecv-creates.mcl

@@ -0,0 +1,11 @@
+exec "a" {
+	shell => "/bin/bash",
+	cmd => "echo hello | tee /tmp/hello",
+	creates => "/tmp/hello",
+}
+
+file "/tmp/hello" {
+	mode => "ug=rw,o=",
+}
+
+Exec["a"].output -> File["/tmp/hello"].content

examples/lang/sendrecv-export.mcl

@@ -0,0 +1,20 @@
+import "os"
+
+$f = "/tmp/export1"
+exec "a" {
+	shell => "/bin/bash",
+	cmd => "echo hello | tee ${f}",
+	creates => "${f}",
+	send_output => os.readfile("${f}"), # extra graph event but safer in debug
+
+	Before => File["${f}"],
+}
+
+file "${f}" {
+	state => $const.res.file.state.exists,
+	mode => "ug=rw,o=",
+
+	Meta:export => ["*",],
+}
+
+Exec["a"].output -> File["${f}"].content